making angle label - spin vertices with python script
The api doc specifies the argument geom as


  geom (list of (bmesh.types.BMVert, bmesh.types.BMEdge, bmesh.types.BMFace))


so 

import bpy
import bmesh
from math import radians
from mathutils import Vector, Quaternion

mesh = bpy.data.meshes .new("Pie")
obj  = bpy.data.objects.new("Pie", mesh)

scene = bpy.context.scene
scene.objects.link(obj)

bm = bmesh.new()

A = bm.verts.new(Vector((0.0, 0.0, 0.0)))
B = bm.verts.new(Vector((1.0, 0.0, 0.0)))
base_line = bm.edges.new((A, B))

result = bmesh.ops.spin(
    bm,
    geom  = [B],
    cent  = A.co,
    axis  = Vector((0.0, 0.0, 1.0)), 
    dvec  = Vector((0.0, 0.0, 0.0)),
    angle = radians(60.0),
    steps = 12,
    use_duplicate = 0
    )   

C = result['geom_last'][0]
bm.edges.new((C, A))

bm.to_mesh(mesh)
bm.free()


works for me. You could also implement this yourself:

def create_arc(bm, start, center, axis, angle, steps):
    angle = angle / steps
    q = Quaternion(axis, angle)

    prev = start
    vec  = start.co - center

    for i in range(steps):
        vec.rotate(q)
        current = bm.verts.new(center + vec)
        bm.edges.new((prev, current))
        prev = current

    return current